Field of the Invention
The present disclosure relates to a distortion correction technique for reducing a distortion of an image.
Description of the Related Art
Due to long-term use of an image forming apparatus including an electrophotographic printer engine and replacement of a component such as a toner cartridge, a characteristic of an optical system (an optical system such as a polygon mirror or an fθ lens involved in laser beam scanning) in the printer engine may change. When the characteristic of the optical system changes, if a different optical system is used for each of process colors, a printing position shift occurs among the process colors. When the printing position shift occurs among the process colors, a color shift is noticeable and the image quality of a printed image thus may deteriorate.
Japanese Patent Application Laid-Open No. 2014-021357 discusses a technique for correcting a printing position shift among process colors to suppress a color shift. An image forming apparatus discussed in Japanese Patent Application Laid-Open No. 2014-021357 prints a chart including an image in which an image having a plurality of parallel lines, which is in reference color from among a plurality of process colors, and an image having a plurality of step-shaped images, which is in adjustment color from among the plurality of process colors, overlap each other. A user views the printed chart, to input one adjustment value representing the degree of a printing position shift between the reference color and the adjustment color. The image forming apparatus uniformly shifts printing positions of the images in the adjustment color based on the input adjustment value to suppress the occurrence of the color shift.
A laser beam for scanning a photosensitive member is curved in a sub-scanning direction or expands or contracts in a main-scanning direction depending on the optical system in the electrophotographic printer engine. The degree of the curvature or the expansion/contraction (i.e., the degree of the printing position shift in the sub-scanning direction and the main scanning direction) differs depending on a position in the main scanning direction. Thus, even if an adjustment value corresponding to one position in the main scanning direction is input, it is difficult to appropriately correct, only with the input adjustment value, a color shift between a reference color and an adjustment color corresponding to another position in the main scanning direction.